09:54 from Waterloo in the late 1970s, which I caught having first taken the 07:22 Macclesfield to Euston (arrive 09:33). Headcode '90' of course, called at Southampton and Bournemouth only, the latter to change engine as you say. On a nice day, time to wander round Weymouth, have some fish & chips, and come back on the return working. I think I only went all the way to Weymouth Quay once, on other occasions just using it for the novelty between London and Southampton (or Bournemouth).
At the time, passenger trains hauled by class 73 locomotives were very unusual, especially during daytime.
EDIT: Even for Weymouth, I did the journey on a normal Macclesfield-Weymouth day return, which was at a time day return tickets for long journeys were still sold.
EDIT: Add picture of 73140 (I think) at Bournemouth having replaced the 33 for the up afternoon journey on 25th. October 1980.